Three kinds of branched fatty acids with two long alkyl chains were synthesized: 2-octadecyleicosanoic acid (I), 3-heptadecyleicosanoic acid (II), and 3,5-dioctadecyloxybenzoic acid (III). The structure assignments were confirmed by IR,1H-NMR and elemental analysis. The monomolecular film properties of the fatty acids at an ai/water interface were investigated. II and III gave stable monomolecular films at a surface pressure of 25 dyn/cm, while I did not. Multilayer films of II and III were grown on silicon substrates, and analyzed by X-ray diffraction.